The present invention relates to a method of manufacturing a silica glass preform of an optical fiber, a light guide, an image fiber, a rod lens, or the like, used in the field of telecommunications or optics.
2. Description of the Related Art
Conventionally, it is known that a silica glass preform of an optical fiber or a light guide used in the field of telecommunications or optics is fabricated by a rod in tube method.
According to the rod-in-tube method, a glass rod serving as a core is inserted to another glass tube used for cladding, and they are heated by heating means such that they are melted into an integral body, thereby forming a silica glass preform. However, this method entails a problem caused by bubbles remaining in the produced preform, which are created due to incomplete crushing of the interface between the glass rod and tube caused during the melting thereof by heating.
As pointed out in general, an optical fiber manufactured from the preform containing bubbles and subjected to heating and drawing, is of a low quality, and accordingly of a low reliability. Especially, such an optical fiber has the problem of low mechanical strength of the optical fiber produced.
As an alternative to the rod-in-tube method, there is proposed a method in which a porous glass body for cladding is formed on the outer circumference of a core glass rod using an isostatic pressing technique, and the produced porous glass preform is dehydrated and consolidated into a silica glass preform (Published Unexamined Japanese Patent Application (PUJPA) No. 61-256937).
The above method when carried out in reality proceeds in the following manner. First, a glass rod serving as a core is inserted into a flexible mold, and the remaining space is filled with silica glass powder as a forming material. Then, the filled mold is compressed from outside by means of a hydrostatic pressure to form a porous glass body on the outer circumference of the glass rod.
Thereafter, the porous glass body thus prepared is removed from the mold, and subjected to a series of process including dehydrating and consolidating to be formed into an optical fiber preform.
In the above-mentioned method, about 0.1 to 0.7 ton/cm.sup.2, for example, of compressing pressure is applied to the mold on the outer surface toward its central axis for about 5 to 40 minutes. In some cases, the glass rod located in the center of the mold, breaks due to the pressure applied. The smaller the outer diameter of the glass rod, the higher the possibility of breakage of the rod. Further, when the glass rod is broken, the molded porous glass body also cracks due to the shock caused by breakage of the glass rod.
As a result, with the isostatic pressing method when employed for manufacturing a porous glass body around an glass rod, there always exists the problem of a low product yield.